1. Field of the Invention
This invention relates in general to scanners using multi-faceted rotating polygon deflectors and in particular to laser printers having the capability to print more than one image size and resolution.
2. Description of the Prior Art
In creating digital images on photosensitive media there are two applications with differing needs. The first application is for large format, high quality images with fine text and graphics in which the production speed is not critical. These images are up to approximately 12 inches in width and use pixel densities over 400 spots per inch. The second application is for smaller format images with minimal text of large font size, and high productivity. In these images, the width is about 5 inches or smaller and the spot density is below about 400 spots per inch. In order to minimize the floor space required by two machines, it is advantageous to use a single printer in which the format and pixel density can be switched easily between the two applications described. In addition, the printer should use a single paper path for both formats and the cost to manufacture such a printer should be lower than that of two separate machines.
The use of laser printers, especially for creating electrostatic images such as in a photocopier, is widely known. In these applications, a single format and single resolution are normally used. High quality laser printers commonly incorporate optical correction means for a defect in the manufacture of polygons called pyramid error, which is a tilt of the polygon facets in such a way that the facets do not form a vertical cylinder. In other words, the facet normals do not lie in the same plane, but can vary from facet to facet. Even small errors of this type cause extensive degradation of images because the error is periodic, recurring with each rotation of the polygon. This can lead to a defect in the image called banding. The human eye is very sensitive to banding errors and it would be desirable to eliminate this type of error.
One of the means for correction of polygon pyramid error is to make the facet conjugate to the image-forming medium. Conjugate means an image of the polygon facet is in focus at the medium. In U.S. Pat. No. 4,040,096 by Starkweather, this conjugation is provided by a cylindrical lens which has power only in the page scan section of the beam. U.S. Pat. No. 4,247,160 by Brueggemann discloses a laser polygon printer which has a positive cylindrical mirror disposed between a polygon and an image-forming medium. However, neither of these devices prints more than one image format.
Printing more than one image resolution with one printer has been done by using a scanner with a reflecting surface which varies in width in the scanning direction. See G. Starkweather in U.S. Pat. No. 3,944,323. Other methods of using more than one image resolution with one printer are shown in the following U.S. Pat. Nos.: 5,289,001; 5,274,492; 5,255,115; 5,239,313; 4,953,036; 4,734,715; 4,578,689. None of these patents disclose the use of multiple polygons and none use more than one scan length.
The use of more than one format is described in U.S. Pat. Nos. 4,651,170 and 4,651,169 by J. Chandler, D. Kessler, and E. Muka. In these patents, the format in the line scan direction, i.e. the scan length, is changed by adjusting the space between the polygon deflector and the image-forming medium. In order to maintain the beam focus on the medium in the scan section of the beam, the optics prior to the polygon deflector are modified. Also, the cylindrical mirror following the polygon is tilted and shifted in order to maintain the polygon mirror facet conjugate with the image-forming medium. The page scan format is changed via a speed change in the moving medium.
A co-pending application for a Dual Format Pre-objective Scanner, Ser. No. 08/771,367, Attorney Docket No. 74,620!, assigned to the same assignee as the present application, shows two different polygons with the same scan objective for scanning separate formats in different resolutions. In that invention, each polygon is shifted into position so that its facet at center of scan is at the same distance from the scan objective and so the deflected beam sweeps through a plane containing the optical axis of the scan objective. This polygon positioning method ensures that the optical conjugate relationship between the polygon facet and image-forming medium is maintained.